In the hardening of a closed curve trace in the form of a ring surface by local heating and subsequent quenching, there is the problem that already hardened regions must not be heated again to a temperature at which the hardness properties are lost again. In the case of a ring surface, an unhardened slip zone, which has for example a width of between 10 mm and 20 mm, is therefore provided between an initial zone, at which the hardening is begun, and an end zone, which is hardened last. In the case of rolling bearings, it is known to relief-grind this slip zone, in order that the rolling bodies do not have bearing contact there. It must also be taken into account that the initial zone and the end zone represent a transitional region with respect to the effective depth of hardening and the hardness at the surface, because both parameters respectively decrease in the direction of the slip zone. In the hardened region between the initial zone and the end zone, on the other hand, uniform hardness properties are achieved as a result of a uniform movement between the hardening device and the workpiece and also as a result of constant parameters. Apart from the speed between the hardening device and the ring surface, the hardening result can be set by the power output and the frequency of the inductor, the amount and temperature of the coolant delivered by the spray and also by controlling any preheating that may be provided.
In the case of the methods for induction surface hardening with an inductor that are known from practice, the hardening device is activated in the initial zone with the parameters intended for continuous hardening and is deactivated again in the end zone.
The documents DE 10 2005 006 701 B3, DE 10 2006 003 014 B1 and DE 10 2008 033 735 A1 disclose methods for induction surface hardening of a ring surface of a workpiece. In the case of this method, a slip zone is avoided or reduced by using two inductors, which are moved counter to one another, starting from an initial zone. The initial zone can in this case be hardened completely, because uniform heating, or at least largely uniform heating, can be achieved by the two inductors. The two inductors are then moved counter to one another and finally come together at the end zone, which on the ring surface lies opposite the initial zone. The end zone is also heated only once, when the two inductors approach one another from both sides. Since the two inductors cannot be brought up unrestrictedly close to one another, the end zone may be initially preheated with a supplementary inductor. A disadvantage of the methods described is that, particularly in the case of large rolling bearings, very considerable expenditure is necessary with regard to the hardening device with counter-running inductors.